Embodiments of the invention relate generally to CT imaging and, more particularly, to a phantom for spectral CT image system calibration.
Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis that ultimately produces an image.
Generally, the x-ray source and the detector assembly are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. The detector assembly is typically made of a plurality of detector modules. Data representing the intensity of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The data are ultimately processed to form an image.
Conventional computed tomography (CT) systems emit an x-ray with a polychromatic spectrum. The x-ray attenuation of each material in the subject depends on the energy of the emitted x-ray. If CT projection data is acquired at multiple x-ray energy levels or spectra, the data contains additional information about the subject or object being imaged that is not contained within a conventional CT image. For example, spectral CT data can be used to produce a new image with x-ray attenuation coefficients equivalent to a chosen monochromatic energy. Such a monochromatic image includes image data where the intensity values of the voxels are assigned as if a CT image were created by collecting projection data from the subject with a monochromatic x-ray beam.
A principle objective of energy sensitive scanning is to obtain diagnostic CT images that enhance information (contrast separation, material specificity, etc.) within the image by utilizing two or more scans at different chromatic energy states. A number of techniques have been proposed to achieve energy sensitive scanning including acquiring two or more scans either (1) back-to-back sequentially in time where the scans require multiple rotations of the gantry around the subject or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials.
High frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two or more energy sensitive scans may be obtained in a temporally interleaved fashion rather than two separate scans made several seconds apart as typically occurs with previous CT technology. The interleaved projection data may furthermore be registered so that the same path lengths are defined at each energy level using, for example, some form of interpolation. Spectral CT data facilitates better discrimination of tissues, making it easier to differentiate between materials such as tissues containing calcium and iodine, for example.
It is important that spectral CT system provide material density images that are accurate. Accordingly, spectral CT systems need to be calibrated to meet the accuracy specifications for different material images. Known calibration methods for the material domain include creating individual material phantoms for a variety of materials and separately analyzing each one. These individual material phantoms are generally created just prior to calibration and are discarded after calibration due since they cannot be stored. The phantom created for one material may vary from the phantom created for the same material at a different time or by a different technician. Additionally, it may be difficult to calibrate a spectral CT system for different patient sizes using such phantoms.
X-ray or CT phantoms for non-spectral CT imaging systems can be made to last a long time. Such phantoms typically comprise synthetic materials configured to mimic the x-ray attenuation of clinically relevant materials such as iodine, fat, water, calcium, and the like in Hounsfield units (HU). However, these synthetic material phantoms fail to accurately mimic the same materials in spectral CT imaging systems. For example, polytetrafluoroethylene (PTFE) or a similar material has been used in the image domain to simulate the HU range of calcium. In the material domain, PTFE fails to mimic calcium.
Therefore, it would be desirable to design a phantom for spectral CT that overcomes the aforementioned drawbacks.